Toroidal-OAM Hybrid Beam for Divergence-Suppressed OAM Wireless Communication

Abstract

Orbital angular momentum (OAM) beams offer spatially independent multiplexing in wireless communications, but their donut-shaped intensity diverges with mode order, causing power capture loss at finite apertures and unequal link budgets. We propose a toroidal-OAM hybrid beam that combines the angular confinement and strong longitudinal electric field of toroidal electromagnetic pulses from a coaxial horn with a conventional OAM beam, lowering angular spread and producing non-zero on-axis intensity. A far-field model is built as a weighted superposition of OAM uniform circular array (UCA) radiation, a super-Gaussian toroidal angular envelope, and a zeroth-order Bessel longitudinal field, controlled by three dimensionless parameters (toroidal confinement, coupling strength, longitudinal coefficient) tied to measurable hardware. Numerical results at 3 GHz with a $5\lambda$ aperture show 12--41% divergence reduction for OAM modes $\ell =1$--$4$, while modes $\ell \ge 5$ become fully center-peaked. The suppression ratio stays unchanged for fixed $kR$, $\kappa$, $\alpha$, $\beta$, and $\ell$. Link evaluation shows 6.4--30.9 dB received SNR gain, over 108% extension in fixed-efficiency link-budget range, and up to five orders of magnitude reduction in multi-mode power imbalance. The central null is filled to 8--100% of the peak; the hybrid beam does not mix OAM modes, with intended-mode power fraction above $0.54$ where the ring is well-defined. OAM mode-division multiplexing capacity at $z=500\lambda$ shows up to $2.1\times$ ($107\%$) capacity enhancement. Full-wave simulation in ANSYS HFSS corroborates the model, confirming the hybrid launcher preserves the OAM vortex phase while reducing divergence compared with a UCA-only baseline. These findings establish toroidal beam engineering as a practical tool for OAM-based wireless links.